1.12 Eclipse Seasons

Pre-Lecture Reading 1.12

Figure 1

•
The Earth-Moon plane is tilted with respect to the Sun-Earth plane by ≈5°.
- •
  Consequently, eclipses do not occur every new and full moon.
•
The intersection of the Earth-Moon plane and the Sun-Earth plane is called the line of nodes.
•
Eclipses occur when both:
- •
  1. The line of nodes points toward the sun.
  - •
    This is called an eclipse season. It is a 38-day period of time when you can, but do not necessarily, have eclipses.
  - •
    2 eclipse seasons = 1 eclipse year
- •
  2. The moon is on the line of nodes.

•
The Earth is nearly spherical and consequently precesses slowly compared to its rotation rate: 26,000 years vs. 1 day.
•
The Earth-Moon system is not spherical and consequently precesses quickly compared to its "rotation" rate: 18.6 years vs. 1 month.
•
Since the precession is in the opposite direction, we call it regression, or regression of the line of nodes.
•
Since the line of nodes regresses, it points to the sun earlier.
- •
  Consequently, the eclipse year is shorter than the tropical year.
- •
  1 eclipse year ≈ 346 days

•
Eclipse seasons: Between 2 and 3 per calendar year.
•
Eclipses (solar + lunar): Between 4 and 7 per calendar year.
•
Eclipses (solar + non-penumbral lunar): Between 2 and 7 per calendar year.
•
Solar eclipses (total + partial): Between 2 and 5 per calendar year.
•
Total solar eclipses: Between 0 and 2 per calendar year.
•
Lunar eclipses (total + partial + penumbral): Between 2 and 5 per calendar year.
•
Non-penumbral lunar eclipses (total + partial): Between 0 and 3 per calendar year.
•
Total lunar eclipses: Between 0 and 3 per calendar year.

Do Question 9.